1. Field of the Invention
The present invention relates to a method of making superplastically formed and diffusion bonded articles and the articles made thereby. The method and articles are particularly well suited for stiffened cellular panel structures useful for hollow airfoils, wing panels, duct work, cases (e.g. the flanged tube forming part of the casing of the engine) and frames (i.e. part of the bearing support) for example. It is especially suited, in certain embodiments, to the manufacture of compound curvature airfoil structures such as compressor or fan rotor and stator airfoils.
2. Description of the Background Art
Superplasticity is the flow characteristic, analogous to that exhibited by a viscous fluid, demonstrated by certain metals which exhibit unusually high tensile elongations without necking, i.e. with uniform reduction in cross-sectional area when elongated, within limited temperature and strain rate ranges. This phenomenon, peculiar to titanium alloys and to certain other metals and metal alloys, has been exploited for producing a variety of articles, especially those having intricate and complex shapes with small radii of curvature.
It is further known that at these same superplastic forming temperatures the same materials can be diffusion bonded with the application of pressure at contacting surfaces. Diffusion bonding is a process which forms a metallurgical bond by the application of heat and pressure to metallic pieces held in intimate contact for a specific length of time. Bonding is thought to occur by the movement of atoms across adjacent faces of the pieces and is a function of time, temperature and pressure. The process is unique in that it allows metals to be joined without significantly changing their physical or metallurgical properties at the joint and with minimum geometrical distortion.
The fabrication of articles by various combinations of superplastic forming and diffusion bonding steps began in the early 1970's in response to the need for light weight, high strength and stiffness airfoils, to reduce the disc rim load, and also for ducts, frames and similar structures, particularly for aircraft and spacecraft. In one early technology, still in use today, the workpieces were bonded only at selected sites, and bonding was prevented at nonselected locations by a coating of maskant, or stopoff. This was necessary to enable the workpieces to be shaped by superplastic forming without bonding at such locations. The early use of maskants is exemplified in U.S. Pat. No. 3,920,175.
The known maskants, of which Boron nitride and Yttrium oxide are the most common, produce contamination which can seriously impair the integrity of the resulting bonds. Embrittlement can also result. In complex structures, especially hollow core structures that employ cellular stiffening cores, it is impossible to totally remove the maskant. Furthermore, since the maskants are typically brushed onto the areas to be precluded from bonding by hand, areas of erratic and inconsistent bonding are likely to occur. Hence the use of maskants is also limited to relatively simple structures and wide joint areas. Contamination is also likely to occur with maskants, preventing reliable bonding.
The serious problems associated with maskants were recognized in the art at least as early as 1976 in U.S. Pat. No. 4,087,037 (see in particular Col. 1 lines 20-42 and lines 55-58; see also U.S. Pat. No. 4,304,821, Col. 1, lines 45-56) which describes a method and press machinery for producing superplastically formed and diffusion bonded articles without the necessity for maskants. The patent proposes avoiding the necessity for maskants by using a complex press capable of sequentially controlling the process to allow completion of the superplastic forming steps before the parts are allowed to contact each other for diffusion bonding. This is achieved in part through the use of a limiting die and a mating flexible die, it being necessary to pressure-form before reaching diffusion bonding temperatures. Note that the patent teaches the necessity of avoiding accidental contact of surfaces not to be bonded, as undesired bonding may lead to significant damage (Col. 3, lines 29-34). Although the patent suggests the ability of the disclosed machinery to form large area structures with compound curvature, applicant is not aware of any significant commercial use of such machinery (or for that matter of any machinery or methodology) for that purpose.
Another approach to avoiding maskants has been to seam weld two or more metal sheets together in a pattern of bonds, and then to superplastically form a honeycomb of connected cells by inflating the welded sheets at temperature, sometimes bonding to external sheets at the same time. Such welding processes are shown for example in U.S. Pat. Nos. 4,351,470, 4,304,821, and 4,217,397, the earliest of which was filed in 1978.
A seam welded bond pattern has certain drawbacks, however, in that it cannot be controlled accurately in enough detail to achieve uniform width bonds, particularly for detailed configurations, and also necessitates a relatively wide bond width leading to undue fracturing stress when the side walls of adjacent cells formed in the metal sheet have to double back on themselves during inflation to meet in the middle of the weld lines. Also, seam welding tends to produce unreliable bonding because the gaps left intentionally between the welds may undergo diffusion bonding, at temperature, thereby precluding the desired fluid communication necessary to achieve uniform superplastic forming. As a result welded structures have in general not been desired for highly stressed parts, particularly for the fabrication of critical parts such as fan airfoils U.S. Pat. No. 4,351,470, referred to above, mentions briefly that "instead of welding, the sheets could be fixed together by some other means, for example by diffusion bonding." [sentence spanning Col. 2-3.] Nevertheless, it does not explain how to achieve such a pattern of diffusion bonding. Since it was recognized that care must be taken to avoid contact at surfaces other than those to be bonded (see, e.g. U.S. Pat. No. 4,087,037, Col. 3, lines 29-34; U.S. Pat. No. 4,304,821, Col. 1, Lines 39-42), presumably the quoted sentence contemplated the use of stopoff or spacers (which pose problems similar to stopoff and additional problems as well) for that purpose.
In addition to the problems posed by the use of maskants and welding and problems ascribed to local thinning of metal during forming (see e.g., U.S. Pat. No. 4,351,470 Col. 1, lines 33-37), problems also arise from the numerous thermal cycles which the structural components undergo in prior processes, which have a debilitating effect on the resulting structure.
Still another serious drawback of certain prior art methods, particularly (but not exclusively) those employing stopoff, is that they make it impossible to inspect and assess the integrity of the bonds after they are formed, since the bonded portions are rendered inaccessible by the fabrication process. This problem is particularly acute when the cellular structure is used as a hollow core for an airfoil, where the cellular structure is sealed as it is formed between the two outer skin layers. Since a large fraction of the cost of the finished structure may be associated with the airfoil itself, as opposed to the stiffening core, such processes are often not economically feasible due to the high reject rate.
Despite the prior attempts to achieve a commercially feasible method of superplastically forming and diffusion bonding articles without maskants or welding, and without using cumbersome and expensive equipment, applicant is unaware of any commercially successful result to date. The lack of a successful method is particularly noteworthy with respect to the manufacture of intricately shaped compound curvature airfoil structures as well as complex structural members such as frames or the structural elements of such members.